EP1371644B1 - Process for the preparation of caprolactam from polyamide containing waste - Google Patents

Abstract

Production of caprolactam from polyamide-containing waste by depolymerizing the polyamide and distilling off and crystallizing caprolactam comprises recycling at least part of the product to the depolymerization step while its permanganate number is less than 10000 seconds and its UV transmission is less than 85%.

Description

The present invention relates to a continuous and economical process for the preparation of highly pure polyamide waste caprolactam suitable for the polycondensation of polyamide 6 (polycaprolactam).

For clean, unassembled, brand new polyamides, a material preparation for products with quality features as they are new, is easily possible. If, however, consumer goods that have been used for years or reuse products from a plastic collection, this can only be done with high technical, energetic and kostenmäßig cost.

This cleaning and sorting operations are necessary to obtain pure material, since plastic products are always adapted to the application and therefore contain manufacturers and manufacturing due additives, colorants, stabilizers, Glasfasem, etc., which complicate a preparation. Therefore, with an economic reuse of polyamide waste, there is no way to disassemble the monomers to produce a product that is indistinguishable from articles made from monomers in the synthetic route.

Caprolactam is the monomer used to make polyamide 6, which is used to make parts for vehicles and injection molded parts using glass fibers and other aggregates, and to spun fibers for yarns that are used to make carpets, carpeting or other everyday items.

A large part of this material is supplied to the intended use in the plastic circuit and goes through the recycling of plastic for recycling. Since the polyamide 6 products are usually not the only material containing amide groups, but also polyamides are present, consisting of different diacids and diamines, must be an identification and sorting in the individual polyamide product groups. If such a separation does not occur, then considerable difficulties are to be expected from mixed reactions of the various monomer groups, yield losses by by-products and inadequate quality in the resulting monomer. A large part of the polyamide 6 plastic, for example, in the production of fibers and especially in the application for floor coverings. The polyamide 6 is spun into fibers which in carpet factories are spun onto a backing fabric, e.g. Polypropylene fleece, needled and processed into loop or suede depending on the type of carpet. After needling the yarn withdrawn from spools on the carrier, the Polfasem is fixed by an adhesive layer to which a polymer foam backing is then applied, which is filled with fillers, such as chalk. The foam backing fulfills the one purpose of the carpet to give a good impact sound insulation and on the other hand to produce the necessary weight so that it rests plan without gluing on the floor.

The treatment of polyamide 6 waste according to its intended use has been described in various patents such as DE 19719734, EP 0681896, US 5598980, DE 2416573, and DE 2507744, and includes the detection and sorting of non-polyamide 6 ware, their comminution and the Separation of dirt and foreign substances. US 5656757 describes a process in which a distillation residue is recycled to the depolymerizer. In JP 2000 03877, caprolactam is recycled after the depolymerization step. In IN 142150, the mother liquor of the crystallization is recycled to the distillation. For the separation of individual components, such as those incurred in the crushing of carpets in the form of foam backing, pile fiber and carrier, preferably multi-stage swimming / sinking methods are used, as a result, by means of centrifuges and salt solutions of different density, a floating of the lighter

Fraction, such as polypropylene fabric, and a sinking of heavier filled carpet backing is effected. A polyamide 6 fraction obtained by this process is freed by washing of pendant salt freight and dried in a continuous process and fed to the depolymerization via an extruder. In the depolymerization, for example, by the method known from DE 887199, EP 0875504, DE 910056, US 4605762, and JP 53-13636 by means of phosphoric acid, a cleavage of the polymer chains and steam introduced drives the resulting caprolactam as a water vapor mixture.

Furthermore, from US 5990306 a process for the preparation of purified caprolactam from a polyamide-containing carpet is known in which after the depolymerization, the caprolactam is distilled and crystallized.

In addition, from US 4107160 a process for the continuous recovery of caprolactam from polycaprolactam waste is known, among other things, a caprolactam-water mixture, which is obtained in the distillation of caprolactam, mixed with fresh water vapor and is fed as an overheated mixture of the depolymerization ,

The recovery of high purity caprolactam from polyamide 6 wastes, as occurs, for example, from used carpets or the other products mentioned above, is subject to constant variations in the composition of the input stream. Thus, on the one hand, the secondary constituents of a carpet, such as foam backing or carrier fabric, differ depending on the manufacturer and, on the other hand, the polyamide 6 is mixed differently with additives, dyes, etc. Therefore, there is a non-uniform and continuous supply of raw materials. From a material with such sometimes extremely fluctuating input products to produce a high purity product that meets or exceeds the quality requirements of the synthesized caprolactam, has not been reached on a large scale.

A disadvantage of the known processes for the treatment of polyamide 6 wastes, therefore, is that they do not provide high-purity caprolactam which is useful for the production of polyamide 6.

The object of the present invention is therefore to provide a process for the production of caprolactam from polyamide-containing wastes, which is cost-effective and easy to carry out and with which a high-purity polyamide 6 and polycondensation to a high-grade and a caprolactam produced from industrial unrecognizable product suitable caprolactam can be produced.

• a) Depolymerisieren der Polyamid-haltigen Abfälle, wodurch ein Caprolactam-Rohmaterial und gegebenenfalls ein Nebenbestandteile bzw. Zuschlagsstoffe enthaltender Strom erhalten wird,
• b) wenigstens einmaliges Destillieren des Caprolactam-Rohmaterials, und
• c) wenigstens einmaliges Kristallisieren des in Stufe b) erhaltenen Caprolactam-Materials, wodurch Caprolactam erhalten wird,

wobei wenigstens ein Teil des in Stufe c) erhaltenen Caprolactams mit einer Permanganatzahl von < 10000 sec. und einer UV-Transmission von < 85 % den Polyamid-haltigen Abfällen vor und/oder während der Depolymerisation zugesetzt wird.The solution to the problem is a process for the production of caprolactam from polyamide-containing wastes comprising the steps

• a) depolymerizing the polyamide-containing wastes, thereby obtaining a caprolactam raw material and optionally a stream containing minor constituents or additives,
• b) at least one distillation of the caprolactam raw material, and
• c) at least one crystallization of the caprolactam material obtained in step b), whereby caprolactam is obtained,

wherein at least a portion of the caprolactam obtained in step c) having a permanganate number of <10000 sec. and a UV transmission of <85% is added to the polyamide-containing wastes before and / or during the depolymerization.

With particular preference the process according to the invention is carried out continuously.

The polyamide-containing material is preferably selected from the group consisting of polyamide-containing moldings, such as parts for vehicles, polyamide-containing injection molded parts with glass fibers and other additives and polyamide-containing fibers, carpets, carpet flooring and other daily articles containing polyamide Life, like clothes. The polyamide-containing material which is fed to the depolymerization, may also contain non-polymers and extraneous additives.

The polyamide-containing wastes are preferably first sorted before carrying out the depolymerization, so that they contain predominantly polyamide 6 and the remaining other substances are separated, as described above.

In addition, the polyamide-containing wastes are preferably compacted and melted before depolymerization.

The depolymerization is preferably carried out at a temperature of about 180 to about 300 ° C and pressures of about 0.5 to about 2 bar, as in DE 887199, EP 0 875 504, DE 910056, US 4605762, US 5990306 and JP 53- 13636 described. In addition, it is preferable to add a basic or acidic catalyst such as phosphoric acid. The reaction conditions during the depolymerization are preferably adapted to the changing composition of the raw material fed to the process, namely the polyamide-containing waste. Thus, the steam temperature can be increased, larger amounts of steam can be used, the molar ratio of phosphoric acid to polyamide can be changed and / or the pressure can be increased or decreased.

After depolymerization, at least one distillation is carried out in the process according to the invention, as also described with reference to FIGS. 1 and 2. The terms "distillation" and "distillation" are used herein generally for distillation, fractional distillation and purification steps, as used in columns, for example in vacuum columns, extraction columns, thickening columns, rectification columns and refraction columns (see FIGS. 1 and 2).

Preferably, at least four distillations are carried out, wherein in the first distillation, as in apparatus C01, a concentration of the crude lactam / water mixture takes place. After passing through several separation and refining stages, the crude caprolactam concentrate passes into a second distillation, such as into the evaporation apparatus C03, where water is driven off and further concentration takes place. Here, the addition of alkali neutralizes acidic components. The third distillation, such as the distillation unit C04, has the task lower than caprolactam to drain boiling constituents with the addition of ammonia-containing nitrogen. After leaving the third distillation, as in the C04 distillation unit, the crude caprolactam passes into the fourth distillation, such as the C05 distillation unit, where distilled pure caprolactam is removed overhead under reduced pressure. However, since this does not yet satisfy the criteria for a high-purity product, it is supplied to the at least one crystallization, such as the fractional crystallization K01 / K02.

In addition, the escaping from the depolymerization in the vapor phase Spaltcaprolactam-water mixture of a first distillation (evaporation) can be supplied in which the escaping and high-energy vapors can be used, resulting from the polyamide 6 granules extraction and still containing monomers and oligomers water to heat and evaporate, the high-energy vapors are condensed and returned as a return to evaporation. The vapors from the first caprolactam distillation, which still contain caprolactam according to the vapor pressure equilibrium and are only partially condensed, can thus be used. The partial condensation in the amount of the required amount of the reflux ratio and the use of the energy content of the remaining Vapors in the depolymerization is advantageous. In addition, the vapor from the polyamide extraction concentration, as in device C05, can be recycled to the depolymerization.

The vapors escaping from the extract water evaporation C02 and the crude caprolactam distillation C03 can also advantageously be supplied to a superheater. The resulting superheated steam can be fed to the cleavage of the polymer chains of depolymerization.

In particular, it has proved to be economically advantageous to recycle a distillation residue before and / or into the depolymerization, which originates from a (vacuum column) distillation apparatus for purifying the caprolactam which is connected downstream in the production process of the caprolactam from polyamide-containing waste of the depolymerization and which, in addition to short-chain polymers, Colorants and products from side reactions still usable amounts of caprolactam, as described below with reference to FIGS. 1 and 2.

This procedure has proven to be particularly advantageous if the depolymerization is preceded by melting of the polyamide-containing material and the melting of the polyamide-containing waste takes place under vacuum. Another method has then proven to be economically more advantageous in terms of polymer degradation and energy consumption over that previously described when feeding a residue from a vacuum column (distillator means for caprolactam purification) during reflow to a location in an extruder For melting, the vapors going into vacuum are not polluted. This can be done for example in the compression zone. The fed residue from the vacuum column on the one hand down the melt viscosity, on the other hand promotes the degradation of the polymer chains and thus facilitates the transport through the extruder at lower driving force.

Preferably, at least two crystallization steps are carried out in the process according to the invention, wherein both steps are preferably carried out fractionally. In this case, the desired product, namely high-purity caprolactam, is deposited first and, after an analysis, either stored until the production of polyamide or fed to a repeated depolymerization, distillation and / or crystallization, as described below with reference to FIGS. 1 and 2.

In the process according to the invention, a fractional crystallization-melting process is used at least once.

It is furthermore advantageous that the crystallized caprolactam obtained from the at least one crystallization, preferably after a first purification step of the fractional crystallization-melting process, to achieve a sufficiently high quality of a synthesized caprolactam, a continuous analysis such as Permanganatzahl, UV- Transmission, APHA number, alkalinity and volatile base content is subjected.

The caprolactam obtained after stage c) with a permanganate number of <10000 sec. And a UV transmission of <85% (also referred to as recycled caprolactam), preferably with a permanganate number of <8000 sec. And a UV transmission of <82%. added to the polyamide-containing material before and / or during depolymerization is a caprolactam which does not meet the criteria for a high purity product suitable for the production of high quality polyamide 6 polycondensates. The recycling of this caprolactam in the process according to the invention promotes inter alia the degradation of the polyamide-containing material.

The entire caprolactam obtained after stage c) is preferably added with a permanganate number of <10000 sec. And a UV transmission of <85% before and / or during the depolymerization.

In addition, at least a portion of the caprolactam obtained in stage c) with a permanganate number of <10000 sec. And a UV transmission of <85%, preferably a permanganate number <8000 sec. And a UV transmission of <82%, in stage b) are added.

Particularly preferably, the caprolactam obtained from stage c), the crystallization, depending on its purity, can either be fed back to the depolymerization, the distillation or again to the crystallization.

Particularly preferred is caprolactam whose quality features APHA> 5, UV transmission <80, Permanganatzahl <5000 sec, alkalinity> 1mmol / kg, volatile bases> 1 mmol / kg, advantageously reduced to the depolymerization, caprolactam with values APHA> 3, UV Transmission <82%, Permanganatzahl <8000 sec, alkalinity ≥0.5 mmol / kg, volatile bases ≥0.5 mmol / kg is advantageously one of the distillation stages and caprolactam with APHA ≥4, UV transmission ≤85%, Permanganatzahl ≤ 10000 sec, alkalinity ≤0.5 mmol / kg, volatile bases ≤0.5 mmol / kg is preferably fed to the crystallization.

Furthermore, it is preferred in carrying out the process according to the invention secondary constituents and additives of the polyamide-containing material used, such as SBR foam backing, chalk, soot and fabric residues, after depolymerization continuously and by analysis and monitoring of the Spaltsäure- and nitrogen content of the current of the additives Contains controlled and regulated depending on the nitrogen content and catalyst content, such as phosphoric acid auszuschleusen.

It has proved to be advantageous to keep the phosphoric acid content in the discharge between 0.1 mass% and 50 mass%, particularly advantageously between 1 mass% and 25 mass%. The content of nitrogen is advantageously between 0.1 mass% and 10 mass%, particularly advantageously between 1 mass% and 5 mass%.

In particular, the analysis and monitoring of the cracking acid and nitrogen content of the stream containing the aggregates and secondary constituents and leaving the depolymerization is advantageously carried out continuously, since the composition of the raw material fed to the process, namely the polyamide-containing waste, is constantly changing,

In addition, it is preferred that the crude caprolactam concentrate from a first distillation (evaporation C 01) is analyzed by continuous analysis for its quality characteristics such as permanganate number, UV transmission, APHA number, alkalinity and volatile base content.

The crude caprolactam preferably has the following minimum properties: APHA> 4, UV transmission <82, permanganate number <8000 sec, alkalinity> 0.5 mmol / kg, volatile bases> 0.5 mmol / kg. If the values do not correspond to the analyzes of a industrially synthesized and desired caprolactam, it is advantageous that the operating conditions in C01-C05, R02 and K01, K02 and the quality features such as Permanganatzahl, UV transmission, APHA number, alkalinity and Volatile base content are fed to a memory, determined by neural network software correlations of operating conditions and quality characteristics and these are used to control and regulate the distillation (evaporation) in the most economical area. For this purpose, linear and nonlinear higher-order equation systems whose constants are determined by equation solvers are determined. An essential point is also Characteristic maps in which dependencies of the product quality are covered by the process conditions, similarly known from the automotive industry for engine control. Significant features of this control are that, for example, falls below the UV transmission, the steam temperature is reduced, with an increase of alkalinity and volatile base content, the phosphoric acid concentration increases and with an increase in Permanganzahlahl the Spaltdampf- or pressure is reduced in Depolymerisationsreaktor. In the distillation columns C01-C05, depending on the deviation of the criteria to be applied to a high-purity caprolactam, there is a reduction or increase in pressure. An increase in the reflux ratio or a lowering of the temperature are carried out as the color values deteriorate.

In the process according to the invention, it is preferable to add additives and / or oxidants to the caprolactam obtained after stage b) in a refining step. If more than one distillation is carried out in step b), then the additives and / or oxidants may be added at least once to the caprolactam obtained after the first and before the last distillation. Preferably, the additives and / or oxidants are added after the first distillation.

It is particularly preferred that the Rohcaprolactamkonzentrat from the distillation (evaporation C01) to improve the quality characteristics such as Permanganatzahl, UV transmission, APHA number, alkalinity and volatile base content by at least one separator before and after the distillation for treatment with additives and / or In this case, the addition of additives and / or oxidants to Rohcaprolactamkonzentrat in a refining step due to the non-uniform raw material addition of the input product, namely the polyamide-containing waste, monitored by continuous analysis and the amount supplied is controlled. The dosage of an oxidizing agent, such as an aqueous potassium permanganate solution, is a function of the UV transmission and the APHA number and should be designed so that more permanganate is added when the color gets worse.

If the UV transmission falls below the value of 85%, for example, the feed of an adsorbent such as activated carbon, below a limit of 66%, the addition of another aid such as diatom earth occurs.

This control is not constant but proportional to the changes, since the impurities introduced with the raw material determine the quantity to be metered and a constant product quality can only be guaranteed in this way, which is why the product properties are measured. It can be achieved that with the least possible additive the greatest possible effect is achieved and an overdose or underdosing can be avoided as a result of the fluctuating raw material composition, since this again causes costs and product quality deterioration. In the subsequent purification stages, stricter / harsher operating conditions may then be used to compensate for these product quality degradations.

In addition, preferably after stage b), a fixed-bed filtration of the caprolactam obtained can be carried out. If more than one distillation is carried out in stage b), a fixed-bed filtration can be carried out after at least one distillation. In this case, the fixed-bed filtration is particularly preferably carried out after the first distillation. If added to the caprolactam after or during stage b) additives and / or oxidants, a fixed-bed filtration before and / or after their addition can be performed.

For economical use of waste heat and reducing the consumption of water, the vapors from a first or second distillation (evaporation C 03) can only partially and condensed in the amount be ensured so that the return amount is ensured, and the remaining non-condensed and hot vapors can be used after overheating as cracking steam during depolymerization.

In addition, it is preferred in the at least one distillation (evaporation) to feed nitrogen with an ammonia concentration of advantageously 100 ppm to 5%, particularly advantageously 250 ppm to 3%. When more than one distillation is carried out, it is preferable to introduce nitrogen in the third distillation (evaporation C04).

In addition, it is preferred that in a distillation, in particular a fourth distillation (Caprolactamrektifikation C05), low-molecular-weight oligomers which can be cleaved to caprolactam with economic advantage and have an advantageous effect on the yield, the melt for refining or the depolymerization are supplied.

In addition, the liquid stream discharged from the depolymerization can be mixed with the residues from a refining reactor, such as R02, and neutralized with inorganic salts, excess phosphoric acid, foreign polymers and oligomers which can no longer be split into caprolactam, and fed to a neutralization, as described in US Pat Figs. 1 and 2 described below. The residue from the neutralization, which still contains organic constituents, can then be sent for thermal utilization.

By carrying out the process according to the invention, high-purity caprolactam is obtained. By recycling the poor product streams, first, a high yield and, secondly, a high purity product are produced.

Another advantage of the method according to the invention in a preferred embodiment described above, that no live steam is required if all water for steam generation or all steam in the cleavage is recycled within the process.

In a further preferred embodiment described above, the inventive method has the advantage that no waste stream leaves the plant containing a caprolactam-water mixture. Thus, the stream 33 described in FIG. 1 contains molten polyamide / phosphoric acid / chalk / aggregates but no caprolactam. All caprolactam-containing streams can be recycled internally and reprocessed.

Figs. 1 and 2 show schematically an apparatus for carrying out a preferred embodiment of the method according to the invention.

In Figs. 1 and 2 meant: equipment A01 gravity A02 Fixed-bed filters A03 filter A04 Fixed-bed filters C01 Aufkonzentrierungseinrichtung C02 Eindampfeinrichtung C03 Destillationseinrirhtung C04 distillation device C05 distillation device E01 reflow K01 crystallizer K02 crystallizer R01 Depolymerisationreaktor R02 Refining reactor R03 Neutralization stage W01 cooler W02 Evaporator W03 Evaporator W04 superheater V01 / S01 Analyzes of controlled distributors material flows 1 Polyamide 6 waste 2 melt 3 Auschleusung depolymerization 4 Steam Poyalmide 6 extraction / concentration 5 phosphoric acid 6 Crude caprolactam / water 7 steam 8th condensate 9 Crude caprolactam concentrate 10 Crude caprolactam concentrate 11 oils 12 Crude caprolactam concentrate 13 filtered crude caprolactam concentrate 14 additives 15 Additives + crude caprolactam concentrate 16 deposited additive 17 Crude caprolactam concentrate 18 filtered crude caprolactam concentrate 19 lye 20 Water vapor feedback 21 Crude caprolactam concentrate 22 low boilers 23 Nitrogen ammonia stripping gas 24 Crude caprolactam concentrate 25 Crude caprolactam residue 26 distilled, pure caprolactam 27 Precursor caprolactam 28 fractionated caprolactam 29 Caprolactam residue 30 high purity caprolactam 31 Polyamide 6 extract 32 Polyamide Extakt concentrate 33 neutralized discharge

The inventive method will now be explained in more detail with reference to FIGS. 1 and 2, wherein the individual features of the embodiment should be understood that they can be used independently advantageous in the method according to the invention.

The production of the high-purity and polycondensation of polyamide 6 products of the very best color and product quality, which is free of property or use-reducing by-products, is characterized in that the polyamide 6 recovered by mechanical processes shows waste as shown in Figure 1, via the feed 1 to a melt-down extruder E01 be supplied. In this extruder, the polyamide 6 wastes are compacted and melted under mild thermal and mechanical conditions. It has become in the course of operation a production plant proved to be economically advantageous that residue 25 which originates from the vacuum column C05 and which contains not only short-chain polymers, color carriers and products from side reactions still usable amounts of caprolactam, not disposal, either by thermal utilization or other methods, is supplied but recycled as a valuable raw material to increase the yield to the incoming contaminated material and combined with this is added to the going into the depolymerization stream 2. The feed of the residue 25 from the vacuum column C05 can be carried out directly downstream of a melt-down extruder E01 into the feed line 2 or into the depolymerization reactor R01. This procedure has proved to be particularly advantageous when the melting of the polyamide waste takes place under vacuum. Another method has then proven to be economically more advantageous in terms of polymer degradation and energy consumption compared to the previously described, when the supply of the distillation residue 25 from the vacuum column CO5 during reflowing takes place at a location in the extruder E01, that the Vacuum going vapors are not charged, this can be done for example in the compression zone. The fed distillation residue 25 from the vacuum column C05 on the one hand down the melt viscosity, on the other hand promotes the degradation of the polymer chains and thus facilitates transport through the extruder at lower driving force.

The polyamide melt leaving the melting apparatus E01 is fed continuously via the feed line 2 to the depolymerization reactor R01, in which by the known process according to DE 887199, EP 0875504, DE 910056, US 4605762, JP 53-13636 at temperatures between 180 ° C. and 300 ° C. , Pressing 0.5 bar and 2 bar of phosphoric acid 5 and steam 4 is fed, whereby a splitting of the polymer chains into oligomers and monomers takes place. As the relevant literature, for example, Brandrup / Bittner / Michaeli / Menges "The recycling of plastics", S. 513-520 Verlag Carl Hanser Munich Vienna 1995 to Caprolactam is re-formed, which leaves the reactor R01 via the vapor line 6 together with the steam and other volatile constituents.

In the production of polyamide 6 granules by ring-opening polycondensation of caprolactam still remain monomer and low molecular weight components as residues in the polyamide 6 granules and deteriorate the performance properties. By a known to the expert water extraction these products are removed from the polymer. The low molecular weight components and soluble monomers contained in the extract water are present in high dilution and cause high costs when disposed of as waste. Considering the economics of evaporating the low concentration solution and recycling the concentrate, it has been found that this method offers cost advantages over disposal as waste. This advantage is also increased by the fact that the vapors 7 and the exergy contained in the concentration of the caprolactam-water mixture are used economically. This is done by the present process in which these vapors 7 are advantageously shown as shown in Figure 1, the sump reboiler W03 of the column C02 for heating and the distillation of the extract water are supplied. A portion of the condensate 8 from W03 is fed after release of its heat content again as reflux in the concentration of caprolactam in the head of C01 and the rest in the neutralization tank R03 used to prepare the neutralization solution and quenching of the residue stream 3.

It has surprisingly been found, during operation of a polyamide 6 product processing plant, that it is beneficial and advantageous for the product purity of the caprolactam when the concentrate 32 from the extraction and thickening column is fed to the depolymerization reactor R01.

Since the mechanical treatment and separation according to the floating-sink centrifuge technology not 100% pure polyamide 6 and completely eliminated by impurities polymer supplied to the extruder E01 in the example still shares in SBR foam backing and chalk from the processing of carpets, soot, fabric residues and other fillers are contained in the stream 2. It has proved advantageous in the commercial operation of a caprolactam recovery plant to discharge these secondary constituents continuously and by means of analyzes and online measurements of the phosphoric acid content via the residue line 3. Analyzing and controlling the mass to be rejected by analyzes and online measurements of the phosphoric acid content in electricity has proven to be particularly advantageous with regard to the economic operation of such a system, since on the one hand the consumption of phosphoric acid and on the other hand the consumption of alkali to neutralize the residue 3 in reactor R03 could be minimized. Furthermore, by measuring and monitoring the nitrogen content in the stream 3 to be discharged, it was possible to advantageously increase the polymer chain scission and thus the yield of caprolactam compared to a non-nitrogen monitored mode of operation in the range from 5% to 25%. The above-described has proved to be extremely beneficial in the operation of a operated with polyamide 6 wastes caprolactam treatment plant. In contrast to a caprolactam production from pure raw materials in which there is no disruption of the continuous operation due to fluctuating composition of the incoming product stream, the product quality fluctuations resulting from the changing composition of the input material can be compensated and compensated.

The caprolactam-water mixture which has passed into the column C01 is concentrated by fractional distillation, and it has proved to be advantageous that by selecting the column internals, the reflux ratio in the range from 1:10 to 10: 1, particularly advantageously from 1: 2 to 1: 4, a temperature in the range of 150 ° C to 300 ° C and 0.1 bar to 1 bar to minimize the remaining in the caprolactam by-products from the Störstoffbeimengungen. It has been found advantageous in view of the purity of the caprolactam to monitor these minor constituents continuously and by analysis and online measurements and to control the operating conditions such as temperature, pressure and reflux ratio in such a way that the best possible removal of the by-products is achieved. Among other things, this goal is achieved by setting various parameters over an operating period of 1000 hours and passing the values measured online to a microcomputer for evaluation with a neural network program.

The product 9 leaving the column C01 still contains substances dispersed in the aqueous phase. It has proven to be advantageous, for achieving a crude caprolactam with good product properties, to incorporate a separator A01 in the line 10, in which the liquid, sometimes oily admixtures dispersed in the aqueous phase are separated off and used as material stream 11 for further utilization, such as, for example. Combustion be supplied. Furthermore, it was found in the course of plant operation that a passing of the Rohcaprolactamstromes 12 by a fixed-bed filter A02 has an advantageous effect on the color purity.

In the subsequent reactor R02, the treatment of the aqueous solution with additives and oxidizing agents 14 known from commercial caprolactam production from pure raw materials, for example potassium permanganate, takes place to improve the color, the acid number and the double bond content. In contrast to industrial production, which has to master continuous and only slightly fluctuating caprolactam quality and can work with fixed additive additions, the product properties of the caprolactam solution vary due to the sometimes extremely changing composition of the incoming polyamide. To take this behavior into account, the Measurement results of the quality control, which take place for example by FTIR, refractometer, UV spectrometer, used on the column C01, the additive type and Additivzugabemasse to dose so that the least necessary chemicals consumption results in the best possible Rohcaprolactamqualität. After separation of the solids content in the separator A03, the product 17 passes again through a fixed bed replaceable filter A04, which has an advantageous effect on the color purity. For further concentration of the aqueous caprolactam solution, the feed of the stream 18 to an intermediate separation stage of a rectification column C03. Here, by known processes, steam at pressures in the range from 20 mbar to 250 mbar and temperatures in the range from 100 ° C. to 200 ° C., reflux ratio in the range from 1:10 to 2: 1 are taken overhead and there is a concentration of caprolactam with the addition of alkaline neutralizing agents 19 instead. For optimum use of waste heat and the consumption of water to cleave the polyamide, it has proven to be advantageous to condense the vapors 20 of C03 only partially and in the amount that the return amount is guaranteed. The remaining non-condensed and hot vapors 20 are added to the stream 4 after overheating in W04.

The concentrate 21 leaving the bottom of the column C03 is introduced into the middle part of a further refraction column C04, where, under pressures in the range from 0.5 mbar to 50 mbar and temperatures in the range from 115 ° C. to 200 ° C., reflux ratio in the range from 1 : 10 to 2: 1 heavier than water boiling components are separated. It has surprisingly been found that the product properties of the recovered caprolactam improve significantly when in the lower part of the column, a nitrogen stream 23 is supplied, which may include both 100 ppm to 5% ammonia. It has in the operation of a carpet recycling plant as beneficial for achieving a low acid content and good color purity in the Stream 24 proved to stripping in the rectification with ammonia-containing nitrogen.

In the step C05, a further rectification column in which an anhydrous caprolactam melt is fed into the middle section under the conditions known to those skilled in the art at reduced pressure in the range of 0.5 mbar to 50 mbar and temperatures in the range of 115 ° C to 200 ° C, Reflux ratio in the range of 1:10 to 2: 1 caprolactam distilled.

In the bottom of the column remain low molecular weight oligomers, which can be split with economic advantage to caprolactam and advantageously due to the yield as residue stream 25 for reconditioning in the extruder E01 or the reactor R01 be recycled.

The caprolactam vapor 26 is partially precipitated by cooling in the purification stage K01. According to the phase equilibrium, the first precipitated solid has a higher degree of purity than the remaining liquid residual melt.

This type of caprolactam purification in K01 and K02 is characterized by its two-step nature, following the familiar zone and meltdown technologies. It has proven to be advantageous to carry out this crystallization in a tube falling film apparatus as it is known, inter alia, from the company. Sulzer. The fusion process following the known fractional crystallization on the precipitation of crystals on a wall yields a first product which is fed via the line 27 to the crystallization stage K02. Associated with this level is the analyzer S01. This analyzer S01erfüllt, it has been found advantageous in the course of operation, the task to check the product melted down on its optical properties and depending on the quality of the melt this via the valve V01 either the depolymerization reactor R01, the column C04 or C05 to allocate or with appropriate purity a second time the crystallization K01 zuzuleiten. The sequence of the fractional crystallization and melting process is determined by the melting temperature of the caprolactam crystals. High-purity caprolactam has the highest melting point and is first deposited on the walls of the apparatus, the subsequent crystal layers are easily contaminated with mother liquor and crystallize later. The most polluted layers are outside. After falling below the temperature in the crystal layer, for example, 5 ° C below the melting point of pure caprolactam, the flow of mother liquor is stopped by the crystallizer and there is the switch from cooling to heating. Since the contaminated crystals first melt, they drip off, are collected and fed to the second stage K02. After, for example, reaching a temperature of 2 ° C. below the melting point of pure caprolactam in K01, the outflow to K02 is stopped, the heating rate is increased and the pure caprolactam is melted off. This process known from K01 is repeated again in K02, with the difference that the crystals melting off at the beginning are brought back into the crystallization K01, depending on the temperature, into other apparatuses and the main product of crystallization K02. According to the invention, the product which is more heavily soiled at the beginning of the melting operation is fed to the depolymerization reactor R01, the intermediate product of the column C03 and the last product of the column C04 or the purification stage K01. The analysis results and a microprocessor-controlled evaluation and control system decide on the quantities. After the zone melting in K01 of impure product is completed, the melting of the remaining pure crystals 25, which is stored or buffered for the removal by a polycondensation plant. This buffer is advantageous and according to the invention also provided with an analysis device, as used for the division of the pre-melt product 21. She serves the Monitoring the acid number and color purity and regulates the release of the product for further processing in a polycondensation plant. If, for example, the purity is no longer achieved due to storage for too long, this analyzer controls a return of all or only part of the contents to purification stage K01 / K02.

The resulting in the depolymerization R01 liquid stream 3 contains inorganic salts, excess phosphoric acid, foreign polymers and oligomers, which can not be cleaved to caprolactam. Since the product contains more acid than a neutral mixture by operation, it is combined with the residues 12 from the reactor R02 mixed and fed to a neutralization step R03. Here, the addition of so much base that a neutral salt 33 is formed, which is supplied due to its residual organic content of a thermal recovery.

Example 1:

• Ohne Rückführung von Strom 25 Rückstand aus Kolonne C04,
• ohne kontinuierliche Phosphorsäure und Stickstoffüberwachung des Produktstromes 3 aus dem Depolymerisationsbehälter R01,
• ohne Kreislaufführung der Brüden 7 aus Kolonne C01 über W03 und Energieaustausch Polyamidextraktwasser C02,
• ohne Überwachung der Produkteigenschaften 9 nach C02 und keine Rückführregelung der Bedingungen in C02,
• ohne Feststofffiltration A02 und A03,
• ohne Rückführung der Brüden 20 aus Kolonne C03 und Nutzung als Dampf in R01
• ohne Einspeisung von ammoniakhaltiges Strippgas 23 in Kolonne C04
• ohne Qualitätsanalyse S01 im Abschmelzprodukt 27 der fraktionierten Kristallisation K01 und Steuerung der Rückführmengen durch V01 zu E01, R01, C04 und C05

Operation of a production plant according to the method of Figures 1 and 2 with fibrous polyamide 6 recovered from carpets

• Without recycling of stream 25 residue from column C04,
• without continuous phosphoric acid and nitrogen monitoring of the product stream 3 from the depolymerization vessel R01,
• without circulation of the vapors 7 from column C01 via W03 and energy exchange of polyamide extract water C02,
• without monitoring the product properties 9 according to C02 and no feedback control of the conditions in C02,
• without solids filtration A02 and A03,
• without recycling the vapors 20 from column C03 and use as steam in R01
• without feed of ammonia-containing stripping gas 23 in column C04
• without quality analysis S01 in the melting product 27 of the fractional crystallization K01 and control of the recirculation quantities by V01 to E01, R01, C04 and C05

Example 2:

• Rückführung von Strom 25 Rückstand aus Kolonne C04,
• kontinuierlicher Phosphorsäure und Stickstoffüberwachung des Produktstromes 3 aus dem Depolymerisationsbehälter R01,

Operation of a production plant according to the method of Example 1 after installation

• Recycling of stream 25 residue from column C04,
• continuous phosphoric acid and nitrogen monitoring of the product stream 3 from the depolymerization tank R01,

Example 3:

• Kreislaufführung der Brüden 7 aus Kolonne C01 über W03 und Energieaustausch Polyamidextraktwasser C02,
• Überwachung der Produkteigenschaften 9 nach C02 und keine Rückführregelung der Bedingungen in C02,
• Feststofffiltration A02 und A03,

Operation of a production plant according to the method of Example 2 after installation

• Circulation of the vapors 7 from column C01 via W03 and energy exchange of polyamide extract water C02,
• Monitoring of product properties 9 according to C02 and no feedback control of the conditions in C02,
• Solids filtration A02 and A03,

Example 4:

• Rückführung der Brüden 20 aus Kolonne C03 und Nutzung als Dampf in R01
• Einspeisung von ammoniakhaltiges Strippgas 23 in Kolonne C04
• Qualitätsanalysegerät S01 im Abschmelzprodukt 27 der fraktionierten Kristallisation K01 und Steuerung der Rückführmengen durch V01 zu E01, R01, C04 und C05

Operation of a production plant according to the method of Example 3 after installation of

• Return of the vapors 20 from column C03 and use as steam in R01
• Feeding of ammonia-containing stripping gas 23 in column C04
• Quality analyzer S01 in melting product 27 of fractional crystallization K01 and control of recirculation quantities by V01 to E01, R01, C04 and C05

Example 5:

As example 4, changed raw material
Wastes from the production of polyamide fibers Table 1 method raw material Polyamide 6 content mass% By-products In the raw material Yield caprolactam% Permanganate number sec UV transmission% Relative energy consumption% industrial caprolactam - - - > 10000 > 85 - Example 1 * PA6 fiber carpet 90 Chalk, SBR, PP 85% Min. 5000 Max. 8000 Min. 75 Max. 85 100 Example 2 * PA6 fiber carpet 90 Chalk, SBR, PP 86 Min. 6500 Max. 8000 Min. 80 Max. 85 95 Example 3 * PA6 fiber carpet 90 Chalk, SBR, PP 90 Min. 10000 Max. 12000 Min. 85 Max. 90 90 Example 4 PA6 fiber carpet 75-96 Chalk, SBR, PP 91 Min. 25000 Max. 27000 Min. 88 Max. 90 80 Example 5 PA6 fiber casts 99 Oils, finishes 97 26000 89 65 * not according to the invention

Claims (13)

1. A method for the manufacture of caprolactam from polyamide-containing waste, comprising the steps of

   a) depolymerising the polyamide-containing waste, whereby a caprolactam raw material and, where applicable, a flow containing secondary constituents or additives is obtained,

   b) distilling the caprolactam raw material at least once, and

   c) crystallising at least once the caprolactam material obtained in step b), whereby caprolactam is obtained,

   
   wherein at least part of the caprolactam obtained in step c) with a permanganate number of < 10000 sec. and a UV transmission of < 85% is added to the polyamide-containing waste before and/or during the depolymerisation.
2. The method according to claim 1, wherein at least part of the caprolactam obtained in step c) with a permanganate number of < 10000 sec. and a UV transmission of < 85% is added to step b).
3. The method according to one of the preceding claims, wherein it is carried out continuously.
4. The method according to one of the preceding claims, wherein an acidic catalyst is added in step a).
5. The method according to one of the preceding claims, wherein secondary constituents or additives are continuously and through analysis and monitoring of the cracking acid and nitrogen content of the flow, which contains the secondary constituents or additives and is obtained in step a), discharged in a controlled manner.
6. The method according to claim 5, whereby the analysis and monitoring are carried out continuously.
7. The method according to one of the preceding claims, whereby the reaction conditions in step a) are adapted to the changing composition of the polyamide-containing material.
8. The method according to one of the preceding claims, wherein the caprolactam obtained after step b) is examined through continuous analysis for its quality features, such as permanganate number, UV transmission, APHA number, alkalinity and volatile base content.
9. The method according to one of the preceding claims, wherein the caprolactam obtained after step b) is passed through at least one separator to improve the quality features, such as permanganate number, UV transmission, APHA number, alkalinity and volatile base content.
10. The method according to one of the preceding claims, wherein additives and/or oxidation agents are added to the caprolactam obtained after step b).
11. The method according to one of the preceding claims, wherein in step b) more than one distillation is carried out and additives and/or oxidation agents are added at least once to the caprolactam obtained after the first and before the last distillation.
12. The method according to one of the preceding claims, wherein in addition a packed bed filtration is carried out before and/or during step b).
13. The method according to claim 12, wherein additives and/or oxidation agents are added to the caprolactam obtained after and/or during step b) and a packed bed filtration is carried out before and/or after their addition.

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